In the quest of sustainable and environmentally friendly energy technologies, proton conducting fuel cells are promising candidates. These can be made of different inorganic materials, barium indate oxide (Ba2In2O5) being one of them. Its hydrogenated version Ba2In2O5(H2O)X (0 < X < 1), which is obtained at high temperatures and increased humidity,is capable of efficient proton conduction, a crucial prerequisite for its functionality.
It has been shown that bariumindate oxide contains at least two proton sites, named H(1) and H(2), which are covalently bound to different oxygen atoms. The proton conduction mechanism of Ba2In2O5(H2O)X has been demonstrated to be due to rotational and jump-like proton motions. However, these localised motions appear insufficient to fully explain proton diffusion in this material; in fact, exchange of protons between these two sites is expected.
The design of future generations of sustainable fuel cells based on such materials requires an in-depth understanding of its proton diffusion mechanism. To this end, a study comprising inelastic neutron scattering and molecular dynamics simulations to explore proton dynamics in fully and partially hydrated versions of Ba2In2O5(H2O)X was designed. The project was led by Maths Karlsson (Chalmers University, Sweden) and run in collaboration with scientists from ISIS (UK), TU Munich (Germany) and the ILL.
"The proton motions were determined to be mainly rotational in the case of H(1). For H(2), we observed proton transfers between adjacent oxygen atoms", says Michael M. Koza, one of the ILL scientists. The residence times of the protons (i.e. the times they remained in a given position) were on the picosecond scale. Such tiny timeframes are readily accessible via energy-resolved neutron experiments. In order to cover a large range of energy resolutions and momentum transfers, thus allowing for monitoring local and longer distance diffusion, the spectrometers IN16B, IN6 at ILL and TOFTOF at MLZ were used complementarily.
The team also made a somehow unexpected discovery. "Our experiments revealed a third proton site, H(3), in the fully hydrated material. This site actually facilitates interexchange between H(1) and H(2). Moreover, this newly discovered site is the reason we observe long-range proton diffusion in the fully hydrated compound," explains Maths Karlsson.
In addition, the proton conduction mechanism within the Ba2In2O5 species studied was shown to be anisotropic. The experimental data were in excellent agreement with molecular dynamics simulations which provided the trajectories of the protons in question.
The experiments described here are a powerful demonstration of the ability of neutrons, in particular inelastic neutron scattering, to determine (sub)molecular motions with unparalleled time resolution. Such investigations are indispensable for a detailed understanding of the mechanisms underlying tomorrow's solutions for sustainable energy management.
Schematic structure of (a) Ba2In2O5 and (b) fully hydrated equivalent BaInO3H, with the notations used in this work.
Reference: Perrichon, A., Koza, M. M., Evenson, Z., Frick, B., Demmel, F., Fouquet, P., & Karlsson, M. (2023). Proton Diffusion Mechanism in Hydrated Barium Indate Oxides. Chemistry of Materials, 35(17), 6713-6725.
ILL contacts: Michael Marek Koza, Bernhard Frick.
ILL instruments: IN6, IN16B